Make all optional PhoneNumber fields default to None.

The fields italian_leading_zero and number_of_leading_zeros currently behave slightly different from other fields in class PhoneNumber and from their corresponding fields in libphonenumber implementations that use protocol buffers.

These changes will make them both default to None and coerce them both to bool and int respectively when they're set, to bring them in line with how other fields are treated and other implementations work.

This will help interoperability with other implementations.